Standard computer monitors and TV displays are typically based on reproduction of three, additive, primary colors (“primaries”), for example, red, green, and blue, collectively referred to as RGB. Unfortunately, these monitors cannot display many colors perceived by humans, since they are limited in the range of color they are capable of displaying. FIG. 1A schematically illustrates a chromaticity diagram as is known in the art. The closed area in the shape of a horseshoe represents the chromaticity range of colors that can be seen by humans. However, chromaticity alone does not fully represent all visible color variations. For example, each chromaticity value on the two-dimensional chromaticity plane of FIG. 1A may be reproduced at various different brightness levels. Thus, a full representation of the visible color space requires a three dimensional space including, for example, two coordinates representing chromaticity and a third coordinate representing brightness. Other three dimensional space representations may also be defined. The points at the border of the horseshoe diagram in FIG. 1A, commonly referred to as “spectrum locus”, correspond to monochromatic excitations at wavelengths ranging, for example, from 400 nm to 780 nm. The straight line “closing” the bottom of the horseshoe, between the extreme monochromatic excitation at the longest and shortest wavelengths, is commonly referred to as “the purple line”. The range of colors discernible by the human eye, represented by the area of the horseshoe diagram above the purple line, at varying brightness levels, is commonly referred to as the color gamut of the eye. The dotted triangular area of FIG. 1A represents the range of colors that are reproducible by a standard RGB monitor.
There are many known types of RGB monitors, using various display technologies, including but not limited to CRT, LED, plasma, projection displays, LCD devices and others. Over the past few years, the use of color LCD devices has been increasing steadily. A typical color LCD device is schematically illustrated in FIG. 2A. Such a device includes a light source 202, an array of liquid crystal (LC) elements (cells) 204, for example, an LC array using Thin Film Transistor (TFT) active-matrix technology, as is known in the art. The device further includes electronic circuits 210 for driving the LC array cells, e.g., by active-matrix addressing, as is known in the art, and a tri-color filter array, e.g., a RGB filter array 206, juxtaposed the LC array. In existing LCD devices, each full-color pixel of the displayed image is reproduced by three sub-pixels, each sub-pixel corresponding to a different primary color, e.g., each pixel is reproduced by driving a respective set of R, G and B sub-pixels. For each sub-pixel there is a corresponding cell in the LC array. Back-illumination source 202 provides the light needed to produce the color images. The transmittance of each of the sub-pixels is controlled by the voltage applied to the corresponding LC cell, based on the RGB data input for the corresponding pixel. A controller 208 receives the input RGB data, scales it to the required size and resolution, and adjusts the magnitude of the signal delivered to the different drivers based on the input data for each pixel. The intensity of white light provided by the back-illumination source is spatially modulated by the LC array, selectively attenuating the light for each sub pixel according to the desired intensity of the sub-pixel. The selectively attenuated light passes through the RGB color filter array, wherein each LC cell is in registry with a corresponding color sub-pixel, producing the desired color sub-pixel combinations. The human vision system spatially integrates the light filtered through the different color sub-pixels to perceive a color image.
U.S. Pat. No. 4,800,375 (“the '375 patent”), the disclosure of which is incorporated herein by reference in its entirety, describes an LCD device including an array of LC elements juxtaposed in registry with an array of color filters. The filter array includes the three primary color sub-pixel filters, e.g., RGB color filters, which are interlaced with a fourth type of color filter to form predetermined repetitive sequences. The various repetitive pixel arrangements described by the '375 patent, e.g., repetitive 16-pixel sequences, are intended to simplify pixel arrangement and to improve the ability of the display device to reproduce certain image patterns, e.g., more symmetrical line patterns. Other than controlling the geometric arrangement of pixels, the '375 patent does not describe or suggest any visual interaction between the three primary colors and the fourth color in the repetitive sequences.
LCDs are used in various applications. LCDs are particularly common in portable devices, for example, the small size displays of PDA devices, game consoles and mobile telephones, and the medium size displays of laptop (“notebook”) computers. These applications require thin and miniaturized designs and low power consumption. However, LCD technology is also used in non-portable devices, generally requiring larger display sizes, for example, desktop computer displays and TV sets. Different LCD applications may require different LCD designs to achieve optimal results. The more “traditional” markets for LCD devices, e.g., the markets of battery-operated devices (e.g., PDA, cellular phones and laptop computers) require LCDs with high brightness efficiency, which leads to reduced power consumption. In desktop computer displays, high resolution, image quality and color richness are the primary considerations, and low power consumption is only a secondary consideration. Laptop computer displays require both high resolution and low power consumption; however, picture quality and color richness are compromised in many such devices. In TV display applications, picture quality and color richness are generally the most important considerations; power consumption and high resolution are secondary considerations in such devices.
Typically, the light source providing back-illumination to LCD devices is a Cold Cathode Fluorescent Light (CCFL). FIG. 3 schematically illustrates typical spectra of a CCFL, as is known in the art. As illustrated in FIG. 3, the light source spectra include three, relatively narrow, dominant wavelength ranges, corresponding to red, green and blue light, respectively. Other suitable light sources, as are known in the art, may alternatively be used. The RGB filters in the filter sub-pixel array are typically designed to reproduce a sufficiently wide color gamut (e.g., as close as possible to the color gamut of a corresponding CRT monitor), but also to maximize the display efficiency, e.g., by selecting filters whose transmission curves generally overlap the CCFL spectra peaks in FIG. 3. In general, for a given source brightness, filters with narrower transmission spectra provide a wider color gamut but a reduced display brightness, and vice versa. For example, in applications where power efficiency is a critical consideration, color gamut width may often be sacrificed. In certain TV applications, brightness is an important consideration; however, dull colors are not acceptable.
FIG. 4A schematically illustrates typical RGB filter spectra of existing laptop computer displays. FIG. 4B schematically illustrates a chromaticity diagram representing the reproducible color gamut of the typical laptop spectra (dashed-triangular area in FIG. 4B), as compared with an ideal NTSC color gamut (dotted triangular area in FIG. 4B). As shown in FIG. 4B, the NTSC color gamut is significantly wider than the color gamut of the typical laptop computer display and therefore, many color combinations included in the NTSC gamut are not reproducible by the typical color laptop computer display.